Protection of metallic surfaces

ABSTRACT

Improved processes for applying aluminum coatings on substrates wherein aluminum is applied to the surface of the substrate and the aluminum surfaced substrate is then heated to a temperature in the range of 350*-450* C with adjacent aluminum surfaces at least in part in contact with other such surfaces. Commonly such aluminum surfaced substrates are placed in contact with each other either in the form of a closed coil or a stack. The improvement comprises applying to the said aluminum surfaced substrate prior to the heat treatment, a coating of an aqueous solution of an organic acid in an amount sufficient to give a dry coating of from 2 to 50 milligrams per square foot. The preferred organic acids are acetic acid, succinic acid, oxalic acid and malonic acid.

United States Patent [191 Green [451 July 3,1973

[ PROTECTION OF METALLIC SURFACES Rosemary Green, 2 Pannal Ash Grove, Harrogate, England [22] Filed: Oct. 23, 1970 [21] Appl. No.: 83,608

[76] lnventor:

[ 1 Foreign Application Priority Data Oct. 27, 1969 Great Britain ..52580/69 [52] U.S. Cl. 148/6.27, 148/6.3 [51] Int. Cl. C23f 7/06 [58] Field of Search 148/627, 31.5; 29/1962, 197, 197.5; 204/58, 40

[56] References Cited UNITED STATES PATENTS 1,877,569 9/1932 Falkenthal 148/627 2,963,409 12/1960 Ramirez 204/58 1,965,682 7/1934 Wood 148/627 X 2,066,842 1/1937 Lodeesen 148/627 OTHER PUBLICATIONS Canning et a1., Dry-Powder Aluminizing of Steel Stip, Journal of the Iron and Steel Institute, August 1967, pp. 848-853 Primary ExaminerRalph S. Kendall Assistant Examiner-Caleb Weston Attorney-Herbert H. Goodman 5 7] ABSTRACT Improved processes for applying aluminum coatings on substrates wherein aluminum is applied to the surface of the substrate and the aluminum surfaced substrate is then heated to a temperature in the range of 350-450C with adjacent aluminum surfaces at least in part in contact with other such surfaces. Commonly such aluminum surfaced substrates are placed in contact with each other either in the form of a closed coil or a stack. The improvement comprises applying to the said aluminum surfaced substrate prior to the heat treatment, a coating of an aqueous solution of an organic acid in an amount sufficient to give a dry coating of from 2 to 50 milligrams per square foot. The preferred organic acids are acetic acid, succinic acid, oxalic acid and malonic acid.

10 Claims, No Drawings PROTECTION OF METALLIC SURFACES The present invention relates to a method for reducing the tendency for adjacent surfaces of aluminum to stick together when heated, for example in processes for the protection of metallic surfaces by the formation of an aluminum diffusion layer thereon. In particular it relates to processes of the type in which aluminum powder or a powdered aluminum alloy is deposited onto a ferrous metal surface, the powder is compacted on to it and the whole subjected to a heat treatment to cause sufficient inter-diffusion to give a non porous surface consisting mostly of aluminum on the substrate. Such processes will hereinafter be termed compaction aluminizing processes. However the process of the invention is not restricted to such processes and also finds use, for example, in the annealing of coils of aluminum strip or stacks of aluminum sheet.

It has been found that when strips, wires or sheets of aluminum or steel onto which an aluminum diffusion layer is being formed are subjected to a heat treatment in the form of a close coil or stack, the edges of the strip or sheet tend, on removal from the furnace, to be matt in appearance as compared to a bright central region. It has also been found that some sticking may occur between adjacent convolutions of a coil of strip or wire or adjacent sheets which may result in handling difficulties and may detract from the appearance of the strip, and in the case of the heating being a stage in a process for the formation of an aluminum coating on a ferrous base stripping aluminum from one surface of the product in limited regions.

Surprisingly we have found that these disadvantages can be mitigated if before the material having an aluminum surface, whether it is simply a surface layer formed by compaction of the aluminum or alloy powder on to a substrate or by other means or whether it is the natural surface of the material, is formed into a close coil or stack there is applied to the substrate a compound which on heating will oxidize aluminum to Al o or yield a component which can oxidize the aluminum coating on the strip during the diffusion heating and gives no components which detract from the appearance of the finished article or have a deleterious effect on the operation of of the process. Components which do possess these properties with hereinafter be termed harmful."

Accordingly the present invention provides a heat treatment for materials having an aluminum surface which comprises applying to the surface before heating it a coating of a compound which during the heat treatment either will itself lead to or will yield a component which will lead to the presence of a uniform oxide coating on the strip after the heating has been completed and yields no harmful components and the substrate is subsequently subjected to heating in a form in which at least some of its surface is in contact with another aluminum surface. V

Without wishing to be bound by any particular theory we believe that the reduction of sticking between the convolutions of the coil and the improved appearance of the product is as a result of there being formed a substantially more uniform adherent oxide layer of greater thickness on the surface of the strip than the thinner less uniform oxide layers formed hitherto.

Compounds to be employed in the process are those which will result in the formation on the strip of a coating of A1 0 which may also contain oxides of metals contained in the alloy applied to the substrate after the heat treatment. The coating formed should not contain any other metal components or any water soluble materials since these tend to detract from the appearance of the coated substrate and also may result in the necessity of washing the strip to remove any water soluble material. Compounds which we have found to be of particular use are organic acids (desirably those which are water soluble for ease of application.) Specific compounds which are suitable include succinic, oxalic, acetic and malonic acids. Other compounds which may be employed, however, include inorganic compounds such as ammonium nitrate.

The compounds employed are desirable applied to the substrate from aqueous solutions, for example by passing the strip through a bath of the solution or by roller coating. In general it is desirable to incorporate a little surface active agent in the solution to ensure uniform distribution of the compound over the surface. The compound which is to produce the oxide film will normally be applied in sufficient amount to give a dry coating of from 2 to 50 mg. sq. ft. preferably 5-20 optimally about 10 mg. sq. ft.

The process of the invention is particularly useful in compaction aluminizing processes such as the Elphal process in which aluminum or aluminum alloy powder (normally of particle size less than BS mesh) is deposited on to the metal surface. Conveniently the powder is deposited onto a wet metal surface by an electrostatic depositor but electrophoretic or other methods may be employed if-desired. The surface and its wet metal coating is then dried. Normally sufficient aluminum to give a coating of the order of 2 thou thick is applied to the substrate. The aluminum or alloy powder is then compacted on to the substrate by the application of pressure, for example by passing the coated substrate through rollers. This compaction tends to break up the oxide coating always present on aluminum particles to result in effective bonding between the substrate and the powder. The compound which will form A1 0 is then applied to the surface and the whole is then subjected to a heat treatment normally at about 350500C preferably 350 to 450C for several hours to cause the aluminum to satisfactorily bond on to the substrate to produce a surface substantially or pure aluminum or starting alloy. The atmosphere employed is commonly air. One such process of the conventional type not involving the addition of a compound to produce A1 0, is described in detail in Journal of the Iron and Steel Institute, August 1967, pages 848-853.

When the process of the invention is used in conjunction with a compaction aluminizing process it is desirable that the solution applied to the substrate should react with the aluminum powder andso assist in removing the thin oxide coat which is normally present on it. This will result in better adherence of the aluminum to the substrate and tends to promote the formation of a more uniform thick and adherent oxide coat over all of the strip. It is desirable for this reason that the solution applied to the substrate has a pH of less than 4 or greater than 9.

The process of the invention as employed in the production of diffusion coatings of aluminum or steel substrates is illustrated by the following Examples:

EXAMPLE 1 Aluminum powder was applied to a steel strip which was then compacted onto the surface by passing through rolls. This surface was then coated uniformly by roller coater with a solution of 5 percent w/v oxalic acid and dried to give a dry coating weight of oxalic acid of mg. sq. ft. The strip was then coiled under tension and the coil heated to 450C in air to promote bonding of the aluminum to the steel. When the cooled coil was uncoiled no sticking was observed and the surface had a pleasing uniform appearance.

A similar coil not subjected to treatment with oxalic acid had areas of sticking which left unsightly marks. Also clue to non-uniform oxidation of the aluminum a generally unsatisfactory appearance was noted.

EXAMPLE 2 A surface such as described in Example I which had been contaminated by rolling oil was treated as before with 5 percent w/v oxalic acid containing 0.05 percent w/v surface active agent to ensure uniform wetting of the surface. Results similar to those in Example 1 were recorded.

EXAMPLE 3 The process described in Example 1 was repeated using succinic acid in place of oxalic acid. Equally satisfactory results were obtained.

I claim:

1. In an improved process for coating aluminum on a metal substrate wherein aluminum alloy powder is compacted onto a metal substrate and then said powder coated metal substrate is coiled or stacked and then heat treated to .a temperature in the range of 350500C with adjacent aluminum powder coated or aluminum alloy powder coated surfaces in contact with other such surfaces, the improvement with consists in applying to the said aluminum powder coated or aluminum alloy powder coated surfaces after compaction and prior to coiling and to the heat treatment, a coating of an aqueous solution of an organic acid in an amount sufficient to give a dry coating of organic acid of from 2 to 50 milligrams per square foot of surface; said aqueous solution having a pH of less than 4 or greater than 9. t

2. The process of claim 1 wherein said organic acid applied to said surface is in an amount sufficient to give a dry coating of from 5 to 20 milligrams per square foot, and wherein said heat treatment is at a temperature in the range of 350-450C.

3. The process of claim 2 wherein said organic acid is selected from the group consisting of acetic acid, succinic acid, oxalic acid and malonic acid.

4. The process of claim 3 wherein aluminum powder is applied to the surface of a steel strip and then compacted onto the steel strip by passing through rolls and then coated with an aqueous solution of said organic acid and dried and the strip is then coiled and heattreated.

5. The process of claim 4 wherein said aqueous solution of said organic acid has a pH of less than 4.

6. The process of claim 4 wherein said aqueous solution of said organic acid has a pH of greater than 9.

7. The process of claim 1 wherein said organic acid is selected from the group consisting of acetic acid, succinic acid, oxalic acid and malonic acid.

8. The process of claim 7 wherein said organic acid is applied to said surface in an amount sufficient to give a dry coating of from 5 to 20 milligrams per square foot, and wherein said heat treatment is at a tempera ture in the range of 350450C.

9. The process of claim 8 wherein said aqueous solution of said organic acid has a pH of less than 4.

10. The process of claim 8 wherein said aqueous solution of said organic acid has a pH of greater than 9. is =0: 1: 

2. The process of claim 1 wherein said organic acid applied to said surface is in an amount sufficient to give a dry coating of from 5 to 20 milligrams per square foot, and wherein said heat treatment is at a temperature in the range of 350*-450*C.
 3. The process of claim 2 wherein said organic acid is selected from the group consisting of acetic acid, succinic acid, oxalic acid and malonic acid.
 4. The process of claim 3 wherein aluminum powder is applied to the surface of a steel strip and then compacted onto the steel strip by passing through rolls and then coated with an aqueous solution of said organic acid and dried and the strip is then coiled and heat-treated.
 5. The process of claim 4 wherein said aqueous solution of said organic acid has a pH of less than
 4. 6. The process of claim 4 wherein said aqueous solution of said organic acid has a pH of greater than
 9. 7. The process of claim 1 wherein said organic acid is selected from the group consisting of acetic acid, succinic acid, oxalic acid and malonic acid.
 8. The process of claim 7 wherein said organic acid is applied to said surface in an amount sufficient to give a dry coating of from 5 to 20 milligrams per square foot, and wherein said heat treatment is at a temperature in the range of 350*-450*C.
 9. The process of claim 8 wheRein said aqueous solution of said organic acid has a pH of less than
 4. 10. The process of claim 8 wherein said aqueous solution of said organic acid has a pH of greater than
 9. 